Flow meters are used for a variety of applications where it is desired to measure the flow rate or volume of a given fluid or gaseous material. Some meters are inferential meters, meaning that the actual displacement of the liquid or gaseous material is not measured. An inferential meter uses some other characteristic other than actual displacement to measure flow rate or volume. Inferential meters sometimes have advantages over positive displacement meters, including smaller size. However, inferential meters are also sometimes more costly than positive displacement meters since inferential meters often include more complex designs and require supporting electronics to properly operate. Therefore, it may be important to find methods of reducing the cost of an inferential meter so that the cost of using an inferential meter versus a positive displacement meter is minimized if not eliminated.
One example of an inferential meter is known as a turbine flow meter, like that described in U.S. Pat. No. 5,689,071. The turbine flow meter described in this patent measures the flow rate of a fluid or gaseous material by determining the number of rotations of a turbine rotor located inside the flow path of the meter. The meter is comprised of a hollow housing that includes a turbine rotor on a shaft inside the flow path created by the housing. The housing is constructed out of a high permeable material, such as stainless steel.
As material enters the inlet port of the meter, the material passes through the turbine rotors causing the turbine rotors to rotate at a rate that depends on the flow rate of the material passing through the housing. The rotational velocity of the turbine rotor is sensed by a pickoff coil. The pickoff coil is excited by an a-c signal that produces a magnetic field. As the turbine rotor rotates, the vanes on the turbine rotor pass through the magnetic field generated by the pickoff coil, superimposing a pulse upon the carrier waveform of the pickoff coil. The superimposed pulses occur at a repetition rate (pulses per second) proportional to rotor velocity and hence proportional to the measured rate of fluid flow.
The pickoff coil is countersunk in a port that is drilled into the housing, but the pickoff coil does not reach the inner portion of the housing. Since the housing is constructed out of a high permeable material, the signal generated by the pickoff coil penetrates the housing to reach the vanes of the turbine rotor and superimposed pulses are detectable by the pickoff coil through the housing as well. One method to reduce the cost of this turbine flow meter is to use a lower cost housing material since the housing material comprises a large majority of the material used in the turbine flow meter. However, less costly materials, such as aluminum for example, have a low permeability thereby making it difficult or impossible for the pickoff coil to detect the rotation of the turbine rotor inside the housing.
Therefore, it is desirable to find a technique to use a lower cost, lower permeable material for the housing of the turbine flow meter without disturbing the performance of the pickoff coil.